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Expert's Corner — polyurethane

The (Almost) Perfect Finish

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Water-based finishes have improved quite a bit over the past few years – to the point where they make an excellent finish for just about every woodworker. In particular, they offer a lot of advantages for DIYers and hobbyist woodworkers, especially those working in small shops. You can use a water-borne finish in place of just about any other film finish (varnish, polyurethane, lacquer) on just about any wood surface (furniture, cabinetry, trim work, and flooring). While it can be sprayed on, it's likely that most DIYers and hobbyists will brush it on, which is what I do. What is...

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TIP: Polyurethane and Sealers

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Oil-based polyurethane is a very durable and hard-curing finish. It bonds well to itself, especially if each coat is sanded a little after it has dried well enough so it powders. This creates fine scratches, which improve the bonding of the next coat. It’s a good idea to do this fine sanding between coats anyway to remove dust nibs. But polyurethane doesn’t bond so well over finishes marketed as sealers, especially over sanding sealer. This sealer is good for use under non-polyurethane varnishes because regular alkyd varnishes gum up sandpaper. So to speed production, a sanding sealer can be used...

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TIP: Successfully Spraying Varnish or Oil Paint

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The usual finishes that are sprayed are lacquer, shellac and water-based finish. These finishes dry fast, sometimes too fast in warm temperatures to successfully brush onto large surfaces. Spraying overcomes this problem. Oil-based varnish, including polyurethane varnish and oil paint, can also be sprayed, of course, because any thin liquid can be sprayed. But you need to be aware of a significant difference. Varnish dries much slower than the other finishes, and unlike lacquer, shellac and water-based finish, each coat should be allowed to fully dry before the next coat is sprayed. You may typically spray lacquer, for example, spray...

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Finishes for Wood Floors

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The two key considerations in choosing a wood-floor finish are resistance to scratches and the large surface to be covered. To stand up to abuse, you need a very durable finish, and to avoid filling the room with overspray that will settle and stick to the finish, you need one that dries slowly enough so it can be applied by hand. The two best choices are oil-based polyurethane and water-based polyurethane. Oil-based polyurethane is more durable than water-based, but it has a strong odor that hangs around for several days, and it has a slight orange coloring (usually referred to...

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TIP: Dealing with Bubbles in a Finish

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TIP: Dealing with Bubbles in a Finish

Bubbles in a finish are more likely from brushing than from spraying, though it’s possible to get bubbles in a sprayed finish if you have the air pressure turned up real high. Bubbles are caused by the turbulence created by the brush gliding over the surface much more than from shaking or stirring the finish. The problem is worse if your shop is hot or if the finish and wood are at different temperatures. Because some formulations bubble less than others, especially with varnishes and water-based finishes, you can also switch to another brand, which may reduce the problem. To...

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